Tubular structures



1 Sept. 29,1970 v I H. C.SECORD 3,530,56J1

TUBULAR STRUCTURES Filed Jan. 20. 1967 2 sheets sheet FIG.

INVENTOR H. C. S ECORD BY LLJQ L WM E l-(Lois ATTORNEYS 'H. C. SECORD TUBULAR STRUCTURES Sep 29, 1970 2 Sheets-Sheet 2 Filed Jan. 20,v 196v INVENTOR H. C SEcoRT 5! A L I -LM04:

ATTORNEYS United States Patent 3,530,567 TUBULAR STRUCTURES Herbert Campbell Secord, Little Cheveralls, Markyate, England Filed Jan. 20, 1967, Ser. No. 610,591 Claims priority, application Great Britain, Jan. 24, 1966,

Int. Cl. B23k 31/02 US. Cl. 29--476.5 20 Claims ABSTRACT OF THE DISCLOSURE A method of forming a tubular structure comprising the steps of helically winding at least one metal strip which is curved in plan when flat, in self-overlapping fashion so that each lap has a part-conical disposition and the thickness of the wall of the structure at any point is formed from a plurality of laps, and thereafter flattening the laps one against another to substantially eliminate any gaps therebetween by expanding the tubular structure beyond the yield point of the metal strips.

This invention relates to tubular structures and in particular to line pipe, i.e. large diameter pipe laid in very long lengths for overland or underwater transmission of gas, oil and other fluids at high pressure.

The tubular structures of the present invention are made by winding thin wide coiled metal strip, particularly steel, helically with a plurality of self-overlaps.

In accordance with the present invention a method of forming a tubular structure comprises helically winding at least one metal strip in multi self-overlapping fashion, so that the thickness of the Wall of the structure at any point is formed from a plurality of laps, and causing the laps to be flattened one against the other so that each lap has a stepped cross-section and the structure has a bore free of steps, said flattening being caused by stretching the strip beyond yield.

The laps will be held strongly together by friction generated therebetween by the pressure of the fluid transmitted with or without reinforcement. This may be accomplished, for example, by inserts, special surface treatments, sealants, synthetic adhesives or by a solder-type bond, applied over part or all of each interface of the laps. In general such reinforcement Will be necessary in order to ensure twist stability, to obviate leakage, and in some cases to increase crushing and unpressurised pull strength.

It has previously been proposed to make pressure vessel cylinders by helically winding, in a self-overlapping fashion, straight strip which is both sufficiently thin and strong to allow it to be wound with extreme tension onto a rotating collapsible mandrel so that the strip accommodates itself to a part-conical disposition by stretching differentially within its elastic limit. These demands on the properties of the strip and the method of winding render its use impracticable for commercial production of large diameter line pipe. These requirements may be circumvented in the present invention by employing curved strip, i.e. strip of substantially uniform Width with one edge longer than the other so that the strip has a curved form when laid flat. The extent by which one edge is longer than the other corresponds to the difference between the helical paths described by the respective edges at the skin and bore of the pipe. The actual skin to bore edge length ratio required to permit essentially gapless part-conical contact between all laps without significant differential elastic deformation is in fact:

sin tH-cos 0 3,530,567 Patented Sept. 29, 1970 where D is the outer skin diameter, d is the inner bore diameter, and 0 is the helix angle defined as the angle which the inner bore edge of the strip makes with the axis of the tubular structure.

The curved strip may be obtained by roller stretching substantially uniform gauge strip so as to be slightly hollow wedge shaped in cross-section across the strip width, i.e. straight strip is differentially stretched between co-operating rollers so that the stretching action increases plastic elongation progressively across the width of the strip which thereby becomes curved. The cross-section of such strip satisfies the equation:

where t is the thickness of the strip at any distance W from the inner shorter edge where the thickness is T, and R is the radius of curvature of the strip in plan to its shorter edge. When such curved strip is coiled, it assumes a slightly frusto-conical form with small gaps between successive laps at the outer edge.

Self-overlapping without appreciable winding tension allows the pipe to be its own mandrel, a snug wind at a constant diameter being ensured by correctly relating the speed of winding with the speed at which the tubular structure is withdrawn from the winder.

The tubular structure may be expanded beyond yield after being wound, in order to flatten the laps and so eliminate substantially the helical steps on the bore and skin, and to establish autofrettage pressure ensuring tight contact between all laps.

An embodiment of the invention will now be described with reference to the drawings which accompany this specification and in which:

FIG. 1 shows a section of pipe wall as wound,

FIG. 2 shows the same wall after expansion.

FIG. 3 is a plan view of part of a length of curved steel strip which is shown flat, the curvature being much exaggerated, and

FIG. 4 is a cross-section on the line IVIV of FIG. 3, with the thickness greatly exaggerated relative to the width.

The horizontal scale of FIGS. 1 and 2 is reduced in relation to the vertical scale by a factor of about 5, thus exaggerating the thickness of the strip relative to its width in order to improve the clarity of the figures. The section of 24-inch diameter pipe shown in FIGS. 1 and 2 has a 0.75" wall thickness, and is wound with a single steel strip which is 72" wide and /8" thick to give six layers 12 at a helix angle of 80.6". It will be seen that flattening has the effect of restoring each lap to a uniform thickness.

A simplified theoretical analysis gives the following stability condition for self-overlapping pipe relying exclusively on inter-lap friction:

Nrrd 2 K +3 21r(N-1) where ,u is the effective coeflicient of friction K is the number of strips N is the number of layers d is the inner wall diameter and W is the strip width.

Even with a single strip 72 wide, for pipes above about 12" diameter it is apparent that friction of strip as nor- 3 mally rolled is not sufficient for stability. Reinforcement will thus normally be required over part, or in some cases all, of the inter-lap area. The pipe can be provided with a liner, e.g. of plastic, or of epoxy-type paint.

Simplicity of the winding equipment favours fabrication of the pipe on site. When the pipe can be laid in one continuous length, as for underwater pipe wound on shore or from a lay-barge, or overland pipe wound on a travelling platform, it can be fabricated without any girth joints. In this case, the pipe can be wound by a rotating winding head, drawing the strip from a coaxial uncoiler preeessing to feed the winder. The pipe is withdrawn continuously without rotation and the winding is stopped as each coil is exhausted so that a new coil can be spliced in by interleaving or butt welding with the end of the previous coil. Withdrawal rolls or belts can be used to grip the pipe to prevent rotation and to impart axial travel at the speed relative to rotation needed to give the required helix angle and degree of overlap. Should two or more strips be used, they would be drawn from planetary uncoilers. The strip is tensioned only enough to ensure the minimum of gaps between successive laps, any residual slack being substantially eliminated by coterminous expansion flattening of the pipe.

When laying conditions require relatively short pipe strings, the same type of winder may be used. Alternatively in this case, the pipe can be fabricated by rotating the pipe, drawing the strip or strips from an uncoiler or uncoilers mounted in a stationary position alongside the pipe. Straight strip can be fed into differential roller stretching apparatus to render it curved and thereafter immediately wound into pipe if desired, the strip thus still being curved in plan before winding.

When two or more strips are wound simultaneously, two strips can be sandwiched together to obtain a thicker wall than with a single strip, or strips can be wound successively interleaved one lap apart, or two strips can be disposed side-by-side, the second having its inner edge the same thickness as the outer edge of the first strip if wedge-shaped strip is used.

A pipe can be formed by winding an inner structure and winding thereover an outer structure, one part being wound clockwise and the other anti-clockwise, both being; expansion flattened sequentially and/or together.

The maximum available strip width (now generally about 72") will normally be preferred, since the speed of fabrication is proportional to strip width; but narrower strip may be preferred if it is cheaper in the thickness required.

The number of layers will generally be three to six, depending on the pipes dimensions and duty, and cost considerations. However, many more layers will in some cases be required, e.g. for very large diameter high pressure pipe for gas storage or transport.

A pipe made by winding strip in accordance with the invention is by its nature less prone to fracture than welded pipe. Its multi-layer construction gives it strength equal to the average properties of the metal, whereas solid wall pipe must be designed on guaranteed minimum yield strength. It also imparts a pseudo-ductility at low temperatures, even if the metal is relatively brittle. The marked longitudinal strength and ductility properties of rolled strip moreover act essentially in the direction of the resultant stresses in the strip. The geometry makes castastrophic propogating fractures impossible. Furthermore, since no welding limitations apply, it is possible to use very high strength quenched-and-tempered strip to decrease weight and cost.

What is claimed is:

1. A method of forming a tubular structure comprising the steps of helically winding at least one metal strip which has a substantially uniform width and has one edge longer than the other, in self-overlapping fashion so that each lap has a part-conical disposition and the thickness of the wall of the structure at any point is formed from ia plurality of laps and thereafter flattening the laps one against another to substantially eliminate any gaps therebetween by expanding the tubular structure beyond the yield point of the metal strips.

2. A method according to claim 1 wherein the strip used for winding is formed by differential roller stretching.

3. A method according to claim 1 wherein the thickness of the strip used for winding satisfies the equation:

in which T represents the thickness of the strip at its shorter edge, t represents said thickness at any point spaced from said edge, W represents the distance between said point and shorter edge, and R represnts the radius of curvature of the strip.

4. A method according to claim 1 wherein the grip between the laps is reinforced.

5. A method according to claim 4 wherein the laps are held together by interfacial bonding.

6. A method according to claim 5 wherein the bonding is by a synthetic adhesive.

7. A method according to claim 5 wherein the bonding is by a solder-type material.

8. A method according to claim 1 wherein the laps are held together interfacially by friction alone.

9. A method according to claim 1 in which the tubular structure is expanded to flatten the laps by means of an assembly of balls running circularly on a mandrel and carried in a cage, said balls being brought to bear against the inside surface of the structure which is at the same time withdrawn over the balls so that said balls travel in a helical path with respect to the structure.

10. A method according to claim 1 in which the tubular structure is expanded to flatten the laps by means of annular hydraulic plugs, arranged to travel with the structure and exert a limited radial expanding force and to retract when the pressure is released, in a cyclic operation.

11. A method according to claim 1 in which two strips are sandwiched together in winding.

12. A method according to claim 1 in which the strips are wound successively interleaved one revolution apart.

13. A method according to claim 1 in which two strips are wound side-by-side.

14. A method according to claim 1 in which a coil of the strip is mounted on an uncoiler, capable of rotating coaxially of the tubular structure, and the strip is wound by a rotating winding head, the tubular structure being continuously removed axially without rotation and the rate of withdrawal of the structure is coordinated with the rate of rotation of the winding head to give a predetermined helix angle and number of overlaps.

15. A method according to claim 1 in which two or more strips are wound simultaneously from planetary uncoilers, the tubular structure being continuously removed axially without rotation and the rate of withdrawal of the structure is coordinated with the rate of rotation of the winding head to give a predetermined helix angle and number of overlaps.

16. A method according to claim 1 in which said at least one strip is mounted on at least one uncoiler, mounted in stationary position alongside the pipe, and said at least one strip is wound by rotating the tubular structure to draw said at least one strip from said at least one uncoiler, the tubular structure being continuously removed axially and the rate of withdrawal of the structure being coordinated with the rate of rotation to give a predetermined helix angle and number of overlaps.

17. A method according to claim 1 in which the structure is formed by winding an inner structure and winding 5 thereover an outer structure, one part being wound clockwise and the other counter-clockwise.

' 18. A method of forming a tubular structure comprising helically Winding at least one metal strip in rnulti selfoverlapping fashion, so that the thickness of the Wall of the structure at any point is formed from a plurality of laps, and causing the laps to be flattened one against the other so that each lap has a stepped cross-section and the structure has a bore free of steps, said flattening being caused by stretching the strip beyond yield.

19. A method of forming a tubular structure according to claim 18 in which said laps are bonded together.

20. A method of forming a tubular structure according to claim 18 in which said laps are bonded together by solder-type bonds.

References Cited UNITED STATES PATENTS 6/1929 Dunlap 138155 XR 3/1953 Probst 29-477.3 XR 3/1957 Conrad 29477.3 XR 2/ 1966 Roberts 29-477.3 XR 2/1969 Dofell et a1. 29477.3

JOHN F. CAMPBELL, Primary Examiner 10 R. B. LAZARUS, Assistant Examiner US. Cl. X.R. 

